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Background: Whether blood eosinophil counts and exhaled nitric oxide (FeNO) are associated with important outcomes in mild asthma is unclear. Methods: This question was explored in a pre-specified analysis of a 52week, open-label, randomized, parallel-group trial in patients with mild asthma receiving only reliever inhalers, comparing salbutamol 200µg asneeded, maintenance budesonide 200µg twice-daily with salbutamol as needed, and budesonide/formoterol 200/6µg as-needed. Outcomes were compared between patients with blood eosinophils of <0.15, 0.15-<0.3 and ≥0.3x109/L; FeNO of <20, 20-50 and >50ppb; and a composite score based on both. Results: The proportion of patients randomised to as-needed salbutamol having a severe exacerbation increased progressively with increasing blood eosinophil sub-group (4.1%, 6.5% and 19.5%; p=0.014). There were no significant interactions between either biomarker and the effect of as-needed budesonide/formoterol compared with as-needed salbutamol for either exacerbations or severe exacerbations. However, there were significant interactions between blood eosinophil sub-groups and the effect of maintenance budesonide compared with as needed salbutamol for exacerbations (p<0.001) and severe exacerbations (p<0.001). Maintenance budesonide was more effective than as-needed salbutamol in patients with eosinophils ≥0.3x109/L for exacerbations (odds ratio 0.13; 95% CI 0.05-0.33) and severe exacerbations (0.11; 0.03-0.45). This was not the case for eosinophils <0.15x109/L (odds ratio for exacerbations 1.15; 0.51-1.28 and severe exacerbations 5.72; 0.97-33.6). There was no consistent interaction between treatment response and FeNO or the composite score. Conclusions: In patients with mild asthma the effects of as-needed budesonide/formoterol on exacerbations are independent of biomarker profile, whereas the benefits of maintenance inhaled budesonide are greater in patients with high blood eosinophil counts.

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